1. Introduction
This invention relates to a metal plating solution for the deposition of reflowable, thick deposits of tin lead alloys such as solder over metal surfaces such as cupreous based substrates including copper, brass, bronze and other alloys of copper. The invention also includes methods for accomplishing deposition of such alloys by immersion plating. The invention is especially useful for the manufacture of printed circuit boards
2. Description of the Prior Art
Tin lead alloy having a composition of between 60 and 65 percent tin and 35 to 40 percent lead (solder) is coated onto copper traces in the manufacture of printed circuit boards. The alloy may function as an etch resist and permits attachment of components to a circuit board. For this latter use, the alloy must melt and reflow at a reasonably low temperature to avoid damage to the circuit during the heating of the board and must have an adequate thickness for surface mounting of components.
Conceptually, there are four methods available for coating tin lead alloys onto copper traces in printed circuit manufacture. These methods include (1) hot air leveling, (2) electroless plating, (3) immersion plating and (4) electroplating. The coating method selected for a specific fabrication process is dependent upon the required properties of the alloy deposited on the substrate and the specific circuit fabrication method used. Each coating process has distinct advantages and disadvantages and the metal deposit obtained from each differs from the deposits obtained by other plating methods.
It is known in the art that to be suitable for printed circuit board manufacture, following reflow, a tin lead alloy deposit over copper circuit lines must be pore free, reasonably thick, should possess a uniform cross section and should be bondable to subsequently attached components. It is also known that as metal is deposited onto a substrate from solution in a plating process, the deposit obtained is often not coherent and may contain numerous pores due to the nature of the plating reaction. Pores cannot be tolerated as they permit migration of corrosive chemicals to the surface of the board during subsequent processing steps in the fabrication process. Pores are typically eliminated in a tin lead alloy deposit by heating the board with the alloy deposit to a temperature in excess of the alloy's melting point whereby the deposit melts and pores are eliminated by reassembly of the deposit, a process known in the art as "reflow" of the deposit In addition, a tin lead alloy must be bondable This means the alloy must melt to permit attachment of components to a circuit board To reflow a tin lead alloy and to bond components thereto, the tin lead alloy should melt at a relatively low temperature to avoid damage to the circuit by heating to an excessively high temperature and the deposit must be adequately thick A thin deposit will not reflow and may not contain the minimum mass of metal required to bond a component to a circuit board, particularly if the component is surface mounted on the board. To obtain a low melting tin lead alloy, the deposit desirably has a concentration of tin and lead at or close to the low melting eutectic of tin and lead (approximately 63% tin and 37% lead by weight--i.e., solder).
Electrolytic plating of tin lead alloys is disclosed by Schlabac and Ryder, Printed and Integrated Circuitry, McGRaw Hill Book Company, Inc., New York, 1963 at page 146. Electrolytic plating of tin lead alloys is one method of choice in the industry but suffers certain disadvantages. For example, for electrolytic deposition, a circuit board must be racked as part of an electrolytic cell which is a labor intensive step. In addition, due to uneven current densities as a consequence of a circuit pattern over which the tin lead alloy is to be deposited, the throwing power of the electrolytic bath does not provide desired uniformity and deposits of uneven thickness are obtained. This problem is aggravated further if the circuit board is one having high aspect ratio through holes. Further, deposits of a 60/40 percent tin lead alloy, the alloy of choice, requires close control of the solution composition and operating conditions of the electrolytic cell. Finally, once the tin lead alloy is coated onto the circuit, and the copper not coated with the alloy is etched, the resultant copper circuit traces are discontinuous and damaged circuits are not readily reworked because electrolytic deposition cannot take place over a discontinuous conductive substrate.
Hot air leveling is also a method conventionally used in the art for providing a tin lead deposit over copper circuit lines in printed circuit manufacture. In this process, the circuit board is floated or immersed in a molten tin lead bath. The molten coating obtained is of non uniform thickness and is then made more uniform by passing the board with the tin lead coating beneath a stream of hot air to level the coating or make it uniform. Notwithstanding the use of the hot air stream, the coating obtained is thin at the heel of the hole and non uniform and with high aspect ratio holes, thin in the center of the hole, the step of hot air leveling is time consuming and labor intensive and with high aspect holes, the coating is thin at the edge of the pads.
Electroless plating baths for each of tin and lead, separately, are known in the art. For example, an electroless tin plating bath is disclosed by Warwick and Shirley, The Autocatalytic Deposition of Tin, Trans. Inst. Met. Fin. 58 9 (1980) pp. 9-14 and by Juergen, Chemical Deposition of Tin and Tin Lead, European Institute of Printed Circuits, Deposition for PCB; Conference: Economics and Cost Savings, Conference location: Munich, West Germany, Conference Date: 1983, Monthly number: EIM8506-033415, 1983. A publication showing the electroless codeposition of tin and lead is not known. It is believed that electroless solder plating baths are difficult to formulate because an autocatalytic solution must contain a mixture of tin and lead as well as a complexing agent and reducing agent for each in a single plating solution.
Immersion plating is an electroless plating process, but is given a separate classification by the art. In immersion plating, deposition is by displacement of an elemental metal from a substrate by metal ions in a plating solution while in electroless plating, in accordance with the art accepted definition, plating takes place primarily by autocatalytic reduction of metal ions from solution.
Since immersion plating is by displacement, immersion plating, like electroless plating, does not employ an external electric current but rather is an electrochemical displacement reaction which depends upon the position the substrate metal occupies in the electromotive series relative to the metal to be deposited from solution. Plating occurs when the metal from a dissolved metal salt is displaced by a more active (less noble) metal that is immersed in the solution. Since copper is more noble than either tin or lead, it would appear that copper should not be plated by an immersion process to give a tin lead deposit. However, when complexed, under acidic conditions, the electropotentials of the tin and lead complexes relative to copper reverse making the utilization of immersion plating for the manufacture of printed circuit boards possible However, major limitations in the use of immersion plating for circuit fabrication exist. These limitations include a relatively slow plating rate, difficulty in obtaining a desired alloy and limited deposit thickness. Limited deposit thickness is due to the fact that the immersion plating reaction is self limiting because as the coating builds to full thickness, the metal deposited from solution masks the underlying base metal which functions as the reducing agent and is required for displacement thereby preventing further displacement. Additionally, as the displaced base metal is dissolved in solution, it becomes a contaminant in solution in increasingly high concentration thereby progressively slowing the rate of displacement. Typical deposit thickness for an immersion tin deposit in the prior art is 50 to 100 microinches, mainly because of the foregoing problems in building the deposit to greater thicknesses.
If the above disadvantages with immersion plating could be overcome, there would be advantages to immersion plating over other methods for depositing tin lead alloy in circuit manufacture. Compared to electroless and electroplating, there is no hydrogen generation during the plating process and no concomitant pitting or similar plating discontinuities in the deposit. Also, the immersion plating process is not subject to surface roughness as found in electroplating processes due to "drag-over" from precleaners, anode corrosion and the like. Further, since electroless baths contain both a metal to be plated and a reducing agent, the bath is potentially unstable subject to spontaneous plating whereas, in immersion plating, there is no problem with spontaneous plate-out due to inherent instability of the bath. Moreover, with immersion plating, neither an electrically continuous circuit nor attachments of electrical contacts are required nor is there a need to maintain a precise current. Finally, an immersion deposit is generally uniform in thickness.
In spite of the potential advantages of immersion plating described above, the prior art generally dismissed immersion plating processes for use in printed circuit fabrication because the prior art believed that thick, bondable (solderable) deposits could not be obtained by immersion plating. As stated in Printed and Integrated Circuitry, supra, at page 138, (immersion deposits are) "limited in thickness, porous, and often poorly adherent and, therefore, of limited interest". The self limiting feature and thickness of immersion tin and lead plating procedures were believed to make soldering impossible and consequently, plating baths for immersion deposits of tin and lead were of minimal interest to the art.
In U.S. Pat. No. 4,194,913, incorporated herein by reference, an immersion plating composition is disclosed for deposition of tin lead alloys. In accordance with the teachings of this patent, an immersion plating solution is disclosed comprising stannous chloride in an amount of from 10 to 150 grams per liter of solution, lead chloride in an amount of from 1 to 12 grams per liter, sodium hypophosphite in an amount of from 10 to 100 grams per liter, thiourea in an amount of 40 to 100 grams per liter, hydrochloric acid in an amount of from 40 to 100 milliliters per liter and gelatin in an amount of from 0.1 to 10 grams per liter. In this patent, patentee states that the plating composition of the patent provides a faster plating rate and a deposit of increased thickness compared to that possible using prior art tin lead alloy immersion plating compositions. Though it is believed that improved immersion deposits are obtained using the compositions of said patent compared to those known to the art prior to said patent, the immersion deposits of said patent are nonetheless of inadequate thickness, are not readily reflowed and consequently, are believed to be unsuitable for the commercial manufacture of printed circuit boards
In European published application No. 0 167 949, also incorporated herein by reference, (hereafter the "European Application"), an immersion tin plating solution that may also contain lead is disclosed as consisting of a tin salt of a fluorine containing mineral acid in an amount of from 1 to 50 grams per liter, a lead salt of a fluorine containing mineral acid in an amount of from 1 to 10 grams per liter, a fluorine containing mineral acid in an amount sufficient to provide a pH varying between 0 and 1, and a sulfur containing complexing agent in an amount of from 10 to 400 grams per liter. Though the patent application concentrates primarily on immersion tin plating solutions, an example of a tin lead plating solution is given. However, thick, reflowable deposits of tin and lead are not obtained from the compositions disclosed in the patent and the compositions are believed to be unsuitable for the commercial fabrication of printed circuit boards.